Method for joining a semiconductor chip to a chip carrier substrate and resulting chip package

ABSTRACT

A method for joining a semiconductor integrated circuit chip in a flip chip configuration, via solder balls, to solderable metal contact pads, leads or circuit lines on the circuitized surface of an organic chip carrier substrate, as well as the resulting chip package, are disclosed. The inventive method does not require the use of a solder mask, does not require the melting of the bulk of any of the solder balls and does not require the use of a fluxing agent.

This is a divisional of application(s) Ser. No. 08/189,530 filed on Jan.31, 1994, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention involves a method for joining a semiconductor integratedcircuit chip to a chip carrier substrate and the resulting chip package.

2. Description of the Related Art

In one class of chip packages, one or more semiconductor integratedcircuit chips are mounted face down, in the so-called flip chipconfiguration, onto solderable metal pads on the circuitized surface ofa chip carrier substrate, e.g., a ceramic substrate, via solder balls.Such mounting is achieved by applying solder balls, having a compositionwhich includes, for example, 97 percent (by weight) Pb and 3 percent (byweight) Sn, to contact pads on the circuit-bearing surface of eachsemiconductor chip. In addition, a solder mask is formed on thecircuitized surface of the chip carrier substrate to prevent solder fromflowing beyond the boundaries of the solderable metal contact pads onthis circuitized surface. Then, the solder balls on the chip contactpads are brought into contact with the solderable metal contact pads onthe substrate circuitized surface, and a fluxing agent, often includingchloro-fluoro-carbons (CFCs), is applied to the solder balls in order toeliminate oxide layers which may have formed on the surfaces of thesolder balls and/or solderable metal contact pads. Finally, the solderballs are heated to a sufficiently high temperature, e.g., 330 degreesC., so that they melt and undergo reflow. Upon cooling andre-solidification, the solder balls form firmly adherent, mechanical andelectrical connections between the contact pads on each chip and thesolderable metal contact pads on the chip carrier substrate.

If the chip carrier substrate is, for example, a ceramic substrate, suchas an alumina substrate, then the above-described chip joining processis definitely effective and results in a useful and commerciallyvaluable chip package. That is, neither the presence of the solder mask,nor the reflow process, during the chip joining process is at alladverse to the final chip package. Of course, the use of fluxing agentscontaining CFCs is considered environmentally undesirable, and attemptsare now being made to replace such fluxing agents with fluxing agentswhich do not contain CFCs.

Significantly, a new type of chip package is now being developed whichincludes one or more semiconductor chips mounted in the flip chipconfiguration on an organic chip carrier substrate, such as a polyimidechip carrier substrate. Because such a polyimide substrate is oftenprocessed while in a tape- or roll-like form, and because the bonding ofa chip or chips to such a polyimide substrate is typically achieved viaan automated bonding process, such an organic substrate is oftenreferred to as a TAB (tape automated bonding) substrate or TAB-likesubstrate. Here, the one or more semiconductor chips are mounted facedown, via solder balls, onto solderable metal contact pads or leads orcircuit lines (hereinafter generically referred to as solderable metalcontact pads) on the circuitized surface of the organic chip carriersubstrate.

Unfortunately, the chip joining process used in connection with ceramicchip carrier substrates is disadvantageous when used in connection withorganic chip carrier substrates, such as polyimide chip carriersubstrates. That is, one of the advantages of, for example, polyimidesubstrates is that they are highly flexible, which makes them moreuseful than the relatively rigid ceramic substrates for a variety ofapplications. However, the presence of a solder mask on a polyimidesubstrate substantially reduces the flexibility of the substrate, whichis obviously undesirable. In addition, the relatively high temperaturesneeded to melt and reflow solder balls distorts polyimide substrates andthereby substantially degrades the dimensional stability of suchsubstrates, which makes subsequent processing of such substrates verydifficult, if not impossible. Moreover, these reflow temperatures are sohigh that polyimide substrates are often on the verge of decomposing,while other chip carrier substrates, such as epoxy/glass chip carriersubstrates, do decompose at these temperatures.

Thus, those engaged in the development of chip packages which includeorganic chip carrier substrates have sought, thus far with relativelylittle success, methods for joining chips to organic chip carriersubstrates which do not require the use of solder masks, which do notrequire the melting of solder balls and which do not require the use offluxing agents.

SUMMARY OF THE INVENTION

The invention involves a method for joining a semiconductor chip in aflip chip configuration, via solder balls, to solderable metal contactpads (i.e., solderable metal contact pads, leads or circuit lines) onthe circuitized surface of an organic chip carrier substrate, whichmethod does not require the use of a solder mask, does not require themelting of the bulk of any of the solder balls and does not require theuse of a fluxing agent. The invention also involves the resulting chippackage.

Significantly, in accordance with the inventive method, the compositionof each of the solder balls, which includes at least a first component,and the composition of the upper portion of each of the solderable metalcontact pads on the organic chip carrier substrate, which includes atleast a second component, are chosen in relation to each other so as tosatisfy a specific requirement. That is, the at least first and secondcomponents are chosen in relation to each other so that when broughtinto proximity with one another and heated, they react to form arelatively low melting point composition (hereinafter denominatedRLMPC), such as a eutectic composition, which includes the first andsecond components. Moreover, the first and second components are chosenso that the RLMPC has a melting temperature which is lower than themelting temperatures of the solder balls and of the contact pads on theorganic chip carrier substrate. Consequently, when the solder balls arebrought into contact with the contact pads, subjected to sufficientpressure to break any oxide layers covering the solder balls and/or thecontact pads, and heated to the melting temperature of the RLMPCassociated with the first and second components, a corresponding liquidmelt forms at and/or adjacent to the interface between each solder ball(the bulk of which remains unmelted) and the corresponding contact pad(the bulk of which also remains unmelted) on the organic chip carriersubstrate. It must be noted that only the relatively thin upper portionof each contact pad is chosen to have a composition which participatesin the reaction leading to the formation of the RLMPC. By contrast, theremainder of the contact pad has a different composition which does notparticipate in the reaction, to preclude dissolution of the bulk of thecontact pad, and does not melt at the melting temperature of the RLMPC.Moreover, because the melting temperature of the RLMPC is lower, andtypically much lower, than the temperature needed to melt the solderballs, it follows that the organic chip carrier substrate suffersneither deformation nor decomposition. Upon cooling to room temperature,the solidified liquid RLMPC forms a solid bond between each solder balland the corresponding contact pad. This bond is more than adequate formost purposes, provided the resulting chip package is not subjected totemperatures equal to or greater than the melting temperature of theRLMPC. (However, it must be noted that a solidified RLMPC does tend todissociate very slowly, at room temperature, over a period of months oryears. Therefore, a bond based upon a solidifed RLMPC, while adequatefor a period of months or years, may not be adequate for longer periodsof time.)

In the event the above-described chip package must be subjected toprocessing temperatures which equal or exceed the melting temperature ofthe interfacial RLMPC, then either after solidification, or while stillliquid, the interfacial RLMPC is subjected to further heating at atemperature or temperatures below the melting temperature of the solderballs and of the contact pads. Preferably, this further heating takesplace at a temperature which is even below the melting temperature ofthe RLMPC. The purpose of this heating is to dissociate the RLMPC over arelatively short period of time, e.g., a few hours, and to disperse thesecond component (derived from the upper portion of the contact pad) ofthe RLMPC into and within each solder ball via solid state diffusion.(The term solid state diffusion means that the second component of theinterfacial RLMPC is diffused into and within a solid, rather than aliquid, solder ball. As a consequence, the second component will benon-uniformly dispersed within the solder ball.) Such dispersion mayresult in the second component chemically combining with one of thecompositional components of the solder ball. In any event, once theinterfacial RLMPC is completely dissociated and the second component(non-uniformly) dispersed within each solder ball, upon cooling to roomtemperature the resulting solder ball is firmly adhered to thecorresponding contact pad. Moreover, because each solder ball is usuallymuch larger than the upper portion of the corresponding contact pad, itfollows that the above process only slightly modifies the composition ofeach solder ball. Therefore, the melting temperature of each solder ballis almost unchanged. Consequently, the resulting chip package canreadily be processed at temperatures significantly higher than themelting temperature of the interfacial RLMPC.

Throughout the above discussion, it has been assumed that the organicchip carrier substrate is, for example, a TAB substrate which, itself,is ultimately to be mounted onto a printed circuit card or printedcircuit board. However, printed circuit cards and printed circuit boardsare themselves fabricated from organic materials, such as epoxy/glasscomposites, and therefore the inventive method is useful for mountingsemiconductor chips directly onto printed circuit cards or printedcircuit boards. Therefore, in what follows, the term organic chipcarrier substrate should be interpreted to denote organic chip carriersubstrates such as, for example, TAB substrates, as well as printedcircuit cards and printed circuit boards.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described with reference to the accompanying drawings,wherein:

FIG. 1 displays the Au--Pb phase diagram;

FIG. 2 displays the Pb--Sn phase diagram;

FIGS. 3 (a)-3 (d) depict the steps involved in a preferred embodiment ofthe inventive method; and

FIG. 4 is a front view of a hot air thermode used in conjunction withthe inventive method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The invention involves a method for joining a semiconductor chip in aflip chip configuration, via solder balls, to solderable metal contactpads (i.e., solderable metal contact pads, leads or circuit lines) onthe circuitized surface of an organic chip carrier substrate without theuse of a solder mask and without melting the bulk of each solder ball,as well as the resulting chip package.

As noted above, in accordance with the inventive method, the compositionof each solder ball, which includes at least a first component, and thecomposition of the upper portion of each carrier contact pad, whichincludes at least a second component, are chosen in relation to eachother so as to satisfy a specific requirement. That is, the at leastfirst and second components are chosen in relation to each other so thatwhen brought into proximity with one another and heated, they undergo areaction resulting in the formation of an RLMPC, such as a eutecticcomposition, at and/or adjacent to the interface between each solderball and the corresponding contact pad. Significantly, the first andsecond components are also chosen so that this interfacial RLMPC, whichincludes the first and second components, has a melting temperaturewhich is lower than that of either the solder balls or the contact pads.As also noted above, only the relatively thin upper portion of eachcontact pad is chosen to have a composition which participates in thereaction resulting in the formation of the interfacial RLMPC. Bycontrast, the remainder of the contact pad has a different compositionwhich does not participate in this reaction, to preclude dissolution ofthe contact pad, and does not melt at the melting temperature of theRLMPC.

Significantly, the way in which the first and second components arechosen in relation to each other is by reference to phase diagrams,which are readily found in standard references. Included among these arethe following three reference books, which are hereby incorporated byreference: (1) Hansen and Anderko, Constitution of Binary Alloys(McGraw-Hill, 1985); (2) Elliot, Constitution Of Binary Alloys, FirstSupplement (McGraw-Hill, 1985); and (3) Shunk, Constitution Of BinaryAlloys, Second Supplement (McGraw-Hill, 1985). Thus, for example, asdepicted in FIG. 1, which is the phase diagram for Au--Pb, as found onpage 223 of reference (1), above, there exists a eutectic compositionfor this alloy system at about 84.4 atomic percent Pb and about 15.6atomic percent Au, having a corresponding eutectic (i.e., melting)temperature of about 215 degrees C. Moreover, as is immediately obviousfrom this phase diagram, the melting temperature of Pb is 327 degreesC., while the melting temperature of Au is 1063 degrees C. Therefore, ifeach solder ball had a composition which included only Pb and the upperportion of each contact pad had a composition which included only Au,and if the solder balls and contact pads were brought into contact witheach other, subjected to sufficient pressure to break any oxide layerson the solder balls and/or contact pads and heated to 215 degrees C.,then a eutectic liquid melt, having a composition which included about84.4 atomic percent Pb and about 15.6 atomic percent Au, would form atthe interface between each solder ball and the corresponding contactpad. On cooling and solidification, the interfacial eutectic compositionwould form an effective mechanical and electrical bond between thesolder ball and the contact pad, provided the corresponding chip packagewas not subjected to temperatures equal to or greater than 215 degreesC. (However, as noted above, such a bond would tend to dissociate veryslowly at room temperature, over a period of months or years.)

During the above-described eutectic reaction between Pb and Au, the Pbsolder ball would serve as the source of Pb, but the bulk of the solderball would undergo no melting. In addition, the upper portion of thecontact pad would serve as the source of Au, but the underlying bulk ofthe contact pad would not participate in the eutectic reaction and wouldnot melt at the eutectic temperature.

By analyzing the phase diagrams contained in the above-listedreferences, one can readily find a variety of solder ball compositions,e.g., binary solder ball compositions, and contact pad compositionswhich react to form the interfacial RLMPCs associated with the inventivemethod. For example, a perusal of the Pb--Sn phase diagram, as found onpage 1107 of reference (1), above, which is reproduced in FIG. 2,indicates that for this alloy system, if the amount of Pb is equal to orgreater than 90 atomic percent and the amount of Sn is equal to or lessthan 10 atomic percent, then the corresponding alloy meltingtemperatures are equal to or greater than 300 degrees C. Based upon theAu--Pb phase diagram, discussed above, it follows that by using a solderball having a binary composition which includes at least 90 atomicpercent Pb and no more than 10 atomic percent Sn, and using a contactpad having an upper portion made of Au, that such a solder ball and sucha contact pad will produce a Au--Pb interfacial eutectic compositionupon heating at 215 degrees C., without melting the bulk of the solderball.

Additional useful combinations of solder ball compositions and contactpad compositions are readily found by perusing phase diagrams. Forexample, a perusal of the Pb--In phase diagram, as found on page 855 ofreference (1), above, indicates that this alloy system includes an alphaphase in which the amount of Pb is equal to or greater than 55 atomicpercent and the amount of In is equal to or less than 45 atomic percent,and the corresponding melting temperature is equal to or greater than235 degrees C. In addition, a perusal of the Au--In phase diagram, asfound on page 211 of reference (1), above, reveals that Au and In form arelatively low melting point composition at 154 degrees C. Therefore, byusing a solder ball having a binary composition which includes at least55 atomic percent Pb and no more than 45 atomic percent In, and using acontact pad having an upper portion made of Au, it follows that such asolder ball and such a contact pad will produce a Au--In interfacialRLMPC upon heating at 154 degrees C., without melting the bulk of thesolder ball.

By employing the type of analysis described above, and by perusing theAg--In phase diagram found on page 27 of reference (1), above, it canreadily be shown that using a solder ball having a binary compositionwhich includes at least 55 atomic percent Pb and no more than 45 atomicpercent In, in combination with a contact pad having an upper portionmade of Ag, that heating at 141 degrees C. results in a Ag--Ininterfacial RLMPC, with no melting of the bulk of the solder ball.Moreover, by using the same solder ball in combination with a contactpad having an upper portion made of Au, it can readily be shown (withreference to the Au--Pb phase diagram) that heating at 215 degrees C.results in a Au--Pb interfacial eutectic composition, with no melting ofthe bulk of the solder ball.

Further perusals of other phase diagrams readily yield additional usefulsolder ball and contact pad compositions. For example, a perusal of theSn--In phase diagram, found on page 861 of reference (1), above,indicates that this alloy system includes a phase in which the amount ofSn is equal to or greater than 90 atomic percent and the amount of In isequal to or less than 10 atomic percent, and the corresponding meltingtemperature is equal to or greater than 200 degrees C. As noted above,the Au--In phase diagram exhibits an RLMPC at 154 degrees C., while theAg--In phase diagram exhibits an RLMPC at 141 degrees C. Therefore, byusing a solder ball having a composition which includes at least 90atomic percent Sn and no more than 10 atomic percent In, in combinationwith a contact pad having an upper portion made of either Au or Ag,heating at 154 degrees C. yields a Au--In RLMPC (if the upper portion ofthe contact pad is made of Au), while heating at 141 degrees C. yields aAg--In RLMPC (if the upper portion of the contact pad is made of Ag),without melting the bulk of the solder ball.

A still further perusal of, for example, the Sn--Sb phase diagram, foundon page 1175 of reference (1), above, reveals that this alloy systemincludes an alpha phase in which the amount of Sn is equal to or greaterthan 89.7 atomic percent and the amount of Sb is equal to or less than10.3 atomic percent, and the corresponding melting temperature is equalto or greater than 246 degrees C. A perusal of the Au--Sn phase diagram,found on page 233 of reference (1), above, reveals the existence ofmultiple compositions, i.e., a eutectic composition and pro-eutecticcompositions, at temperatures of 217 degrees C. and higher. In addition,a perusal of the Ag--Sn phase diagram, found on page 52 of reference(1), above, reveals the existence of a eutectic at 221 degrees C.Therefore, by using a solder ball having a composition which includes atleast 89.7 atomic percent Sn and no more than 10.3 atomic percent Sb incombination with a contact pad having an upper portion made of either Auor Ag, heating at 217 degrees or higher yields Au--Sn interfacialmulti-compositional RLMPCs (if the upper portion of the contact pad ismade of Au), while heating at 221 degrees C. yields a Ag--Sn interfacialeutectic composition (if the upper portion of the contact pad is made ofAg), without melting the bulk of the solder ball.

As noted above, each solder ball and contact pad must be subjected tosufficient pressure to break any oxide layers which may have formed onthe surface of the solder ball or contact pad. In general, the amount ofpressure varies depending on the compositions of the solder ball andcontact pad, as well as the size of the solder ball. Therefore, ingeneral, the appropriate amount of pressure must be determinedempirically. This is readily achieved by mounting control samples of thesemiconductor chip of interest, using the solder balls of interest, ontothe contact pads of control samples of the organic chip carrier ofinterest. Then, increasing amounts of pressure are to be applied tosuccessive control samples, while heating these control samples at thetemperature needed to form the desired interfacial, relatively lowmelting point composition. The pressure needed to break the oxide layersis that pressure at which the interfacial melt composition first forms,which is readily detected by taking cross-sections of the solder ballsused with the control samples and examining these cross-sectionalsamples using conventional techniques to detect the presence of theinterfacial melt composition. For example, such detection is readilyaccomplished using conventional scanning electron microscopy techniques,in combination with conventional wavelength dispersive spectroscopytechniques or conventional energy dispersive spectroscopy techniques.

The strength of the mechanical and electrical connection between asolder ball and its corresponding contact pad is proportional to theamount of the RLMPC composition formed at the interface between thesolder ball and the contact pad. Because the volume of the solder ballis usually much larger than that of the upper portion of the contactpad, it follows that the volume of the upper portion of the contact paddetermines the amount of the RLMPC, and therefore the strength of theconnection. In general, the volume of the upper portion of the contactpad needed to achieve a desired strength is determined empirically. Thatis, this determination is achieved by employing control samples of theorganic chip carrier of interest having contact pads with upper portionsof varying volume, mounting control samples of the semiconductor chip ofinterest on the control samples of the organic chip carrier, via solderballs of interest, and then reacting all of the upper portions with thesolder balls. (One can readily determine whether all of the upperportions have reacted with the solder balls by taking cross-sections ofthe contact pads and solder balls and examing the cross-sectionalsamples using conventional scanning electron microscopy techniques.) Byapplying conventional pull tests (tests in which the semiconductor chipis pulled away from the corresponding organic chip carrier substrate, orvice versa) to the control samples of the organic chip carrier, one canreadily determine which control sample exhibits the desired strength.

If a chip package, fabricated in accordance with the above-describedinventive method, must be subjected to further processing, or mustoperate, at temperatures which equal or exceed the melting temperatureof the RLMPC (or RLMPCs) formed at the interfaces between the solderballs and contact pads, then the strength of the mechanical andelectrical connections between these solder balls and contact pads willobviously be compromised. To avoid such compromise, and in accordancewith the invention, a chip package which has been processed as describedabove, is subjected to further heating, either after solidification ofthe interfacial RLMPC, or while this composition is still liquid. Toavoid melting either the solder balls or the contact pads, this furtherheating occurs at a temperature or temperatures below the meltingtemperatures of the solder balls and of the contact pads. Preferably,this heating occurs at a temperature which is below the meltingtemperature of the RLMPC. The purpose of this heating is to completelydissociate the interfacial RLMPC and to disperse the second component ofthe interfacial RLMPC into and within each solder ball via solie statediffusion. As a consequence, the concentration of the second componentwithin the solder ball will be non-uniform. Significantly, thedissociation of the interfacial RLMPC implies that this composition isno longer present, and therefore cannot melt, if the chip package issubjected to temperatures which equal or exceed the melting temperatureof the RLMPC. Moreover, because the solder balls are usually much largerthan the upper portions of the contact pads, the amount of materialdiffused into each solder ball is usually very small in comparison withthe volume of the solder ball. Therefore, the composition of the solderball is only slightly altered during this process, and therefore themelting temperature of the solder ball is only slightly altered. Becausethe melting temperature of the solder ball invariably remains muchhigher than the melting temperature of the interfacial RLMPC, it followsthat the solder ball will not melt when the chip package is exposed totemperatures which exceed the melting temperature of the interfacialRLMPC.

To provide concrete examples of the above-described additionalprocessing, if the composition of each solder ball includes only Pb andthe upper portion of each contact pad is made of Au, then, as describedabove, the interfacial RLMPC will be a Au--Pb eutectic composition.Therefore, the purpose of the additional heating would be to dissociatethe Au--Pb eutectic composition and to disperse the Au (the second)component of the eutectic composition into and within the solder ballvia solid state diffusion. Similarly, if the composition of each solderball includes, for example, Pb and In, and the upper portion of eachcontact pad is made of, for example, Ag, then, as described above, theinterfacial RLMPC will be a Ag--In eutectic composition. Therefore, thepurpose of the additional heating would be to dissociate the Ag--Ineutectic composition and to disperse the Ag (the second) component ofthe eutectic composition into and within the solder ball via solid statediffusion.

In general, the higher the temperature used, the shorter is the amountof time needed to completely dissociate an interfacial composition andto disperse the second component of the interfacial composition into andwithin a solder ball. For a given heating temperature, the amount oftime needed to completely dissociate and to disperse is readilydetermined empirically by heating control samples of the chip package ofinterest at this given temperature for successively increasing times,cross-sectioning the solder ball regions of these control samples andusing conventional scanning electron microscopy techniques incombination with, for example, conventional wavelength dispersivetechniques, to detect the presence or absence of the interfacialcomposition.

It should be noted that when the second component of an interfacialRLMPC diffuses into a solder ball, it may chemically combine with acompositional component of the solder ball. For example, when Audiffuses into a solder ball having a composition which includes Pb andSn, the Au combines with the Sn to form a Au--Sn compound, i.e., AuSn₂.

Significantly, when the second component of an interfacial RLMPC isdispersed via solid state diffusion into and within a solder ball, theresulting solder ball is readily distinguished from solder balls whichhave not been subjected to such solid state diffusion, including solderballs into which the second component has been dispersed via liquiddiffusion. This distinction is readily shown by verticallycross-sectioning a solder ball at its largest diameter and imposing animaginary 10×10 grid of equally dimensioned rectangles on the exposedsurface area of the cross-sectioned solder ball, all of which rectanglesmust fall within, and cover at least 90 percent of, the exposed surfacearea. If one then measures the concentration of the second component (inelemental or compound form) at the center of each rectangle, then asolder ball which has been subjected to solid state diffusion willsatisfy the following criterion:

    ((C.sub.h -C.sub.1)/C(avg))×100>30%.

Here, C_(h) denotes the highest measured concentration, C₁ denotes thelowest measured concentration and C(avg) denotes the average measuredconcentration. It must be emphasized that a solder ball which has beensubjected to liquid diffusion will not satisfy this criterion.

Parenthetically, it should be noted that the concentrations associatedwith the above-described criterion are readily measured usingconventional scanning electron microscopy techniques, in combinationwith conventional wavelength dispersive spectroscopy techniques orenergy dispersive spectroscopy techniques.

As a pedagogical aid to a more complete understanding of the invention,a description of the preferred embodiment of the inventive method, andof the resulting chip package, is given below.

The preferred embodiment of the inventive method is depicted in FIGS.3(a)-(d). In this preferred embodiment, the solder balls 30 mounted onthe contact pads 20 of a semiconductor chip 10 have a composition whichincludes at least 94 percent (by weight) Pb and no more than 6 percent(by weight) Sn, with the preferred composition being 97 percent (byweight) Pb and 3 percent (by weight) Sn. In addition, the largestdiameters of the solder balls are, for example, 0.004 inches or 0.005inches. Moreover, the height of each of the solder balls (which areobviously truncated spheres) ranges from about 50 percent to about 90percent of the largest diameter of the solder ball.

In the preferred embodiment, the semiconductor chip 10 is to be mounted,via the solder balls 30, onto the contact pads 50 on a polyimide chipcarrier substrate 40. Each such contact pad is either circular in shape,with a corresponding diameter of 0.005 inches, or oval in shape, withcorresponding large and small diameters of, respectively, 0.006 inchesand 0.004 inches.

Significantly, as shown in FIG. 3, each contact pad 50 includes a layer60 of Cu (having a melting temperature of 1083 degrees C.), a layer 70of Ni (having a melting temperature of 1453 degrees C.) and a layer 80of Au. Significantly, the layer 60 of Cu is not involved in the eutecticreaction, described below. In addition, the layer 70 of Ni serves as adiffusion barrier, preventing Cu from diffusing upwardly and reactingwith the Au, which would otherwise undesirably reduce the amount of Auavailable for the eutectic reaction. In addition, the layer 70 preventsAuSn₂ (the formation of which is described below) from diffusingdownwardly, to the solder ball/contact pad interface, which would resultin a weak and brittle solder ball/contact pad bond.

The thickness of the layer 60 of Cu ranges from about 0.0005 inches toabout 0.0025 inches, and is preferably 0.0014 inches. Thicknesses lessthan about 0.0005 inches are undesirable because they are mechanicallyunsound. On the other hand, thicknesses greater than about 0.0025 inchesare undesirable because they are unnecessary and do not improve theperformance of the contact pad 50.

The thickness of the layer 70 of Ni ranges from about 0.000015 inches toabout 0.000035 inches, and is preferably about 0.000025 inches.Thicknesses less than about 0.000015 inches are undesirable because theyimply a relatively high probability of voids in the layer 70, whichundermine the barrier function of the layer 70. On the other hand,thicknesses greater than about 0.000035 inches are unnecessary and donot improve the barrier properties of the layer 70.

The thickness of the layer 80 of Au ranges from about 0.000015 inches toabout 0.000035 inches. Thicknesses less than about 0.000015 inches areundesirable because they again imply a relatively high probability ofundesirable voids in the layer 80. On the other hand, thicknessesgreater than about 0.000035 inches result in the formation of anundesirably large amount of interfacial Au--Pb eutectic composition,discussed below, which is difficult to dissociate within convenientperiods of time. Moreover, such a large amount of eutectic compositionincludes so much Au that all of the Au cannot be diffused into thesolder ball.

In the course of mounting the semiconductor chip 10 onto the polyimidechip carrier substrate 40, in accordance with the preferred embodimentof the inventive method, the chip 10 is initially placed face-up on apre-heat stage 100, depicted in FIG. 4. This pre-heat stage, whichincludes a resistance heater (not shown), is used to pre-heat the chip10, and therefore the solder balls 30, to a temperature of 200 degreesC. for 5 to 10 seconds. Then, the polyimide chip carrier substrate 40 isplaced face-down over the chip 10, and the contact pads 50 brought intocontact with the solder balls 30, as depicted in FIG. 3(b).

Significantly, FIG. 4 depicts a hot air thermode 110 which is used inthe preferred embodiment of the inventive method to apply pressure tothe solder balls 30 and contact pads 50, to break any oxide layerscovering the solder balls or contact pads. In addition, the hot airthermode is used to heat the solder balls and contact pads to atemperature greater than 215 degrees, in order to form the Au--Pbeutectic composition, discussed above, at the interfaces between thesolder balls and the contact pads. As shown in FIG. 4, the hot airthermode 110 includes a heat exchanger 120, which includes a resistanceheater (not shown). Electrical power is communicated to the resistanceheater via an electrical conductor 130. The hot air thermode 110 alsoincludes a tube 140, throught which nitrogen gas is flowed into the heatexchanger 120. In addition, the thermode 110 includes a tube 150,through which room temperature nitrogen gas is flowed into the heatexchanger 120. Moreover, the thermode 110 includes a nozzle 160, fromwhich nitrogen gas entering the heat exchanger exits. A temperaturesensor 135 is positioned within the nozzle 160. The end of the nozzle160 includes a layer 170 of sintered, porous metal, which is permeableto the nitrogen gas.

After the polyimide chip carrier substrate 40 is placed over the chip 10and the contact pads 50 brought into contact with the solder balls 30,the layer 170 of sintered, porous metal of the nozzle 160 of the hot airthermode 110 is placed against the back of the chip 10 and sufficientforce is applied so as to subject each of the solder balls to a force of8 grams per chip. This is done for a period of about 0.5 to about 1.0seconds, which is sufficient to break any oxide layers on the surfacesof the solder balls and/or contact pads. Then, nitrogen gas is flowedthrough the tube 140 and into the heat exchager 120, while electricalpower is supplied to the resistance heater in the heat exchanger. As aconsequence, the nitrogen gas is heated and, upon flowing through thenozzle 160, serves to heat the solder balls 30 and contact pads 50 toabove 215 degrees C. This heating is continued for about 0.3 to about1.0 seconds, thereby producing the above-described Au--Pb liquideutectic composition at the interfaces between the solder balls and thecontact pads. Interestingly, this liquid composition tends to get pushedto the sides of each solder ball 30 by the thermode 110. After thisheating is completed, room temperature nitrogen gas is flowed throughtube 150 and out of nozzle 160 for a period of about 1.0 to about 3.0seconds, in order to cool the Au--Pb liquid composition, and therebyform the solid electrical/mechanical connection 90 depicted in FIG. 3(c)between each solder ball 30 and the corresponding contact pad 50.

After the formation of the electrical/mechanical connections 90 betweenthe solder balls 30 and contact pads 50, the resulting chip package isremoved from the pre-heat stage 100 and the solder balls andelectrical/mechanical connections 90 are preferably encapsulated in anepoxy encapsulant. During this encapsulation process, the chip packageis baked at 165 degrees C. for about 3 hours, in order to cure theepoxy. Significantly, this baking process also serves to dissociate allof the solid Au--Pb connections 90 and to disperse the Au into andwithin the solder balls via solid state diffusion, where the Auchemically combines with Sn to form AuSn₂. The results of thisdispersion process are depicted in FIG. 3(d), which shows AuSn₂non-uniformly dispersed in a solder ball 30.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A method for fabricating a semiconductor chip package, comprising the steps of:bringing a region of solder, mounted on a chip contact pad of a semiconductor integrated circuit chip, into contact with a carrier contact pad of a chip carrier substrate, which solder region has a composition which includes at least a first component and a second component and which carrier contact pad includes a pad region having a composition which includes at least a third component; forming a region of material at and/or adjacent to the interface between said solder region and said carrier contact pad, which material region has a composition which includes at least said second component and said third component, while using said solder region and said carrier contact pad as sources of said second component and said third component, said material region having a melting temperature which is lower than that of said solder region and of said carrier contact pad; disassociating said region of material; and diffusing at least said third component of said material region into said solder region via solid state diffusion.
 2. The method of claim 1, still further comprising the step of reacting said diffused third component with at least one component of said solder region.
 3. A method for fabricating a semiconductor chip package, comprising the steps of:bringing a region of solder, mounted on a chip contact pad of a semiconductor integrated circuit chip, into contact with a carrier contact pad of a chip carrier substrate, which solder region has a composition which includes at least a first component and a second component and which carrier contact pad includes a pad region having a composition which includes at least a third component; wherein said first component is Pb, said second component is In and said third component is Ag, and forming a region of material at and/or adjacent to the interface between said solder region and said carrier contact pad, which material region has a composition which includes at least said second component and said third component, while using said solder region and said carrier contact pad as sources of said second component and said third component, said material region having a melting temperature which is lower than that of said solder region and of said carrier contact pad.
 4. A method for fabricating a semiconductor chip package, comprising the steps of:bringing a region of solder, mounted on a chip contact pad of a semiconductor integrated circuit chip, into contact with a carrier contact pad of a chip carrier substrate, which solder region has a composition which includes at least a first component and a second component and which carrier contact pad includes a pad region having a composition which includes at least a third component; wherein said first component is Sn, said second component is In and said third component is AG, and forming a region of material at and/or adjacent to the interface between said solder region and said carrier contact pad, which material region has a composition which includes at least said second component and said third component, while using said solder region and said carrier contact pad as sources of said second component and said third component, said material region having a melting temperature which is lower than that of said solder region and of said carrier contact pad.
 5. A method for fabricating a semiconductor chip package, comprising the steps of:bringing a region of solder, mounted on a chip contact pad of a semiconductor integrated circuit chip, into contact with a carrier contact pad of a chip carrier substrate, which solder region has a composition which includes at least a first component and a second component and which carrier contact pad includes a pad region having a composition which includes at least a third component; wherein said first component is Sb, said second component is Sn and said third component is Au, and forming a region of material at and/or adjacent to the interface between said solder region and said carrier contact pad, which material region has a composition which includes at least said second component and said third component, while using said solder region and said carrier contact pad as sources of said second component and said third component, said material region having a melting temperature which is lower than that of said solder region and of said carrier contact pad.
 6. A method for fabricating a semiconductor chip package, comprising the steps of:bringing a region of solder, mounted on a chip contact pad of a semiconductor integrated circuit chip, into contact with a carrier contact pad of a chip carrier substrate, which solder region has a composition which includes at least a first component and a second component and which carrier contact pad includes a pad region having a composition which includes at least a third component; wherein said first component is Sb, said second component is Sn and said third component is Ag, and forming a region of material at and/or adjacent to the interface between said solder region and said carrier contact pad, which material region has a composition which includes at least said second component and said third component, while using said solder region and said carrier contact pad as sources of said second component and said third component, said material region having a melting temperature which is lower than that of said solder region and of said carrier contact pad.
 7. A method for fabricating a semiconductor chip package, comprising the steps of:bringing a region of solder, mounted on a chip contact pad of a semiconductor integrated circuit chip, into contact with a carrier contact pad of a chip carrier substrate, which solder region has a composition which includes at least a first component and which carrier contact pad includes a pad region having a composition which includes at least a second component; forming a region of material at and/or adjacent to the interface between said solder region and said carrier contact pad, which material region has a composition which includes at least said first component and said second component, while using said solder region and said carrier contact pad as sources of said first component and said second component, said material region having a melting temperature which is lower than that of said solder region and of said carrier contact pad; dissociating said region of material; and diffusing at least said second component of said material region into said solder region via solid state diffusion.
 8. The method of claim 7, further comprising the step of reacting said diffused second component with at least one component of said solder region.
 9. A method for fabricating a semiconductor chip package which includes an organic chip carrier substrate, comprising the steps of:bringing a region of solder, mounted on a chip contact pad of a semiconductor integrated circuit chip, into contact with a carrier contact pad of an organic chip carrier substrate, which solder region has a composition which includes at least a first component and a second component and which carrier contact pad includes a pad region having a composition which includes at least a third component; and forming a region of material at and/or adjacent to the interface between said solder region and said carrier contact pad, which material region has a composition which includes at least said second component and said third component, while using said solder region and said carrier contact pad as sources of said second component and said third component, said material region having a melting temperature which is lower than that of said solder region and of said carrier contact pad.
 10. The method of claim 9, further comprising the steps of:dissociating said region of material; and diffusing at least said third component of said material region into said solder region via solid state diffusion.
 11. The method of claim 10, still further comprising the step of reacting said diffused third component with at least one component of said solder region.
 12. A method for fabricating a semiconductor chip package which includes an organic chip carrier substrate, comprising the steps of:bringing a region of solder, mounted on a chip contact pad of a semiconductor integrated circuit chip, into contact with a carrier contact pad of an organic chip carrier substrate, which solder region has a composition which includes at least a first component and which carrier contact pad includes a pad region having a composition which includes at least a second component; forming a region of material at and/or adjacent to the interface between said solder region and said carrier contact pad, which material region has a composition which includes at least said first component and said second component, while using said solder region and said carrier contact pad as sources of said first component and said second component, said material region having a melting temperature which is lower than that of said solder region and of said carrier contact pad; dissociating said region of material; and diffusing at least said second component of said material region into said solder region via solid state diffusion.
 13. The method of claim 12, further comprising the step of reacting said diffused second component with at least one component of said solder region. 